(U-TH)/HE THERMOCHRONOLOGY AND OROGENY OF THE WASHINGTON CASCADES

Rock uplift, not volcanism, has created most of the topography of the Washington Cascades, especially north of Mt. Rainier. Little is known, however, about the timing, rate, spatial variation, or cause of uplift in the range or in its contiguous neighbors to the north, the Coast Mountains of British Columbia or southern Alaska. Sedimentary and fossil evidence indicates that topographic relief was essentially absent in the Eocene, and that the modern orographic rain shadow did not develop until late Miocene. These observations, as well as uplifted and warped lavas of the Columbia River Basalt Group on the east side of the range suggest most of the uplift postdates 15 Ma. New apatite (U-Th)/He ages from bedrock across a wide area in the WA Cascades show several patterns that constrain exhumation, and, indirectly, uplift history. Despite progressively higher relief and deeper exhumation from south to north along the range, He ages show little north-south variation, and instead show a systematic west-to-east variation across the range. The youngest ages (1-4 Ma) are generally found about 50 km west of the topographic crest, and the oldest ages (45-60 Ma), to the far west or at the crest. Age-elevation-relationships from vertical transects and abundant 6-12 Ma ages in some places suggest a possible late Miocene pulse of exhumation, but He ages can also be modeled reasonably well as representing long-term, average erosion rates. These model erosion rates show a systematic trend and order-of-magnitude variation across the range that closely tracks mean annual precipitation rate. Coupled erosion and precipitation rates and decoupling from topography suggests that precipitation sets the scale and pattern of erosion, and, if the range is in steady state, rock uplift. This would mean that orographic precipitation exerts a strong control on the spatial pattern rock uplift and deformation.